1. Field of the Invention
The present invention relates to a rechargeable lithium battery preferably shaped in a thin form and a process for producing said rechargeable lithium battery.
2. Related Background Art
In recent years, the global warming of the earth because of the so-called greenhouse effect due to an increase in the content of CO2 gas in the air has been predicted. For instance, in thermal electric power plants, thermal energy obtained by burning a fossil fuel is being converted into electric energy, and along with burning of such fossil fuel, a large amount of CO2 gas is being exhausted in the air. Accordingly, in order to suppress this situation, there is a tendency of prohibiting to newly establish a thermal electric power plant. Under these circumstances, so-called load leveling practice has been proposed in order to effectively utilize electric powers generated by power generators in thermal electric power plants or the like, wherein a surplus power unused in the night is stored in rechargeable batteries installed at general houses and the power thus stored is used in the daytime when the demand for power is increased, whereby the power consumption is leveled.
Now, for electric vehicles which do not exhaust any air polluting substances such as CO2, NOx, hydrocarbons and the like, there is an increased demand for developing a high performance rechargeable battery with a high energy density which can be effectively used therein. Besides, there is also an increased demand for developing a miniature, lightweight, high performance rechargeable battery usable as a power source for portable instruments such as small personal computers, word processors, video cameras, and cellular phones.
Under such circumstances, there have been proposed a nickel-metalhydride rechargeable battery and a rechargeable lithium battery which will comply with such demand. And various researches and developments have been made in order to more improve their performances.
For the nickel-metalhydride rechargeable battery, although it is inferior to the rechargeable lithium battery in terms of being relatively heavier, it has advantages in that it can be relatively easily produced at a reduced production cost in comparison with the rechargeable lithium battery. In view of this, nickel-metalhydride rechargeable batteries have been often using as power sources of portable instruments. Besides, nickel-metalhydride rechargeable batteries have started using as power sources of certain electric vehicles.
For the rechargeable lithium battery, there have been proposed various rechargeable lithium batteries having an anode comprising a given anode active material such as a lithium metal, a lithium alloy, a carbonous material, or the like and having a cathode comprising a given cathode active material such as manganese dioxide, lithium-cobalt oxide, lithium-nickel oxide, or the like. These rechargeable lithium batteries have been evaluated as being superior to the nickel-metalhydride rechargeable battery particularly in a viewpoint that they are expected to have a relatively higher energy density. And researches and developments have been made of these rechargeable lithium batteries in order to put to practical use. Some of them have been practically using as power sources of particularly portable instruments.
Incidentally, for the configuration of such rechargeable battery used in a portable instrument, a cylindrical shape or a prismatic shape is adopted in many cases. In the case of a prismatic rechargeable battery, it can be designed to be thinner than a cylindrical rechargeable battery. Thin-shaped prismatic rechargeable batteries have been often using in compact portable instruments.
Now, a cylindrical rechargeable battery is generally prepared in the following manner. A separator is sandwiched between an anode and a cathode such that the separator is partly protruded at each end side, followed by spirally winding about a given axis so as to form a cylindrical body comprising the separator/the cathode/the separator/the anode/the separator. The cylindrical body is inserted in a cylindrical battery vessel through its opening. A necking is formed in the vicinity of the opening of the battery vessel. Then, an electrolyte solution is introduced into the battery vessel so that the separator is impregnated with the electrolyte solution. Thereafter, a capping capable of serving also as an external terminal and which is provided with an internal pressure release vent, a PTC (positive temperature coefficient device), and a current-shutoff device is put on the necked portion of the battery vessel so as to cover the opening, followed by being caulked through a packing. By this, there is obtained a cylindrical rechargeable battery.
A prismatic rechargeable battery is generally prepared, for instance, in the following manner. A separator is sandwiched between an anode and a cathode, followed by winding about a given axis to form a cylindrical body comprising the separator/the cathode/the separator/the anode/the separator. The cylindrical body is shaped into a flat body by means of pressure forming. The flat body is inserted in a prismatic battery vessel through its opening. Then, a capping capable of serving also as an external terminal and which is provided with an internal pressure release vent, a PTC (positive temperature coefficient device), a current-shutoff device, and a liquid introduction port is put on the opening of the prismatic battery vessel, followed by subjecting to laser beam welding to seal the inside of the prismatic battery vessel. Thereafter, an electrolyte solution is introduced into the prismatic battery vessel through the liquid introduction port provided at the capping so that the separator is impregnated with the electrolyte solution. Then, the liquid introduction port is sealed. By this, there is obtained a prismatic rechargeable battery.
Any of the cylindrical battery vessel used in the preparation of the cylindrical rechargeable battery and the prismatic battery vessel used in the preparation of the prismatic rechargeable battery is formed by deep-drawing an appropriate metallic member such as a nickel-plated iron plate, an aluminum plate, or a stainless steel plate.
Particularly in the above method of preparing a prismatic rechargeable battery, it is required to use a relevant prismatic battery vessel, and such prismatic battery vessel is formed by deep-drawing an appropriate metallic member such as a nickel-plated iron plate, an aluminum plate, or a stainless steel plate. In this case, there is a limit for the metallic member which can be processed to form such prismatic battery vessel by way of deep-drawing. Specifically, in the case of using a metallic member such as a nickel-plated iron plate, an aluminum plate, or a stainless steel plate, a prismatic battery vessel formed by way of deep-drawing unavoidably becomes to have a relatively large thickness of about 5 mm or more. This situation is similar also in the case of forming a cylindrical battery vessel by way of deep-drawing.
In order to form a prismatic battery vessel having a thickness which is thinner than aforesaid thickness, it is considered to adopt a method of first forming a prismatic battery vessel by way of deep-drawing and grinding the walls of the prismatic battery vessel in the thickness direction. However, this method results in a remarkable increase in the production cost of a prismatic rechargeable battery and therefore, it is not acceptable in practice.
Separately, when the battery vessel in any case is of a thin thickness, the capping is necessary to have a thin thickness accordingly. When the capping is of a thin thickness of, for instance, less than about 5 mm, it is extremely difficult to work a terminal cap and an insulating mold at the capping and it is also extremely difficult to work a liquid introduction port at the capping. In addition, the battery vessel and the capping are welded by means of laser beam welding in many cases. The welding in this case exerts a thermal adverse effect to the insulating mold situated in the vicinity of the position where the welding is conducted.
Incidentally, in recent years, there has been developed a so-called sheet type rechargeable battery capable of being thinned, comprising a battery main body covered by a laminate film, wherein said battery main body comprises an ion conductor disposed between an anode and a cathode, said ion conductor comprising a separator impregnated with an electrolyte solution, a gelated electrolyte or a solid electrolyte. However, this sheet type rechargeable battery has disadvantages such that because the laminate film is insufficient in terms of the physical strength, the battery is liable to deform or it is liable to be damaged, and therefore, there is a limit for a range where the battery can be used.
FIGS. 10(a) and 10(b) are schematic views illustrating a rechargeable lithium battery having an armor comprising a laminate film, as an example of aforesaid sheet type rechargeable battery.
Particularly, FIG. 10(a) is a schematic perspective view of said rechargeable lithium battery when viewed from the lateral direction, and FIG. 10(b) is a schematic cross-sectional view of a peripheral portion of said rechargeable lithium battery, taken along the line D—D, and when viewed from above.
In FIG. 10(a), reference numeral 1001 indicates a pair of power output terminals extending from a battery main body 1003 installed in a pack formed using a laminate film 1005. The battery main body 1003 comprises an ion conductor disposed between an anode and a cathode.
As will be understood with reference to FIG. 10(b), the laminate film 1005 comprises an aluminum foil 1007 (having a thickness of, for instance, about 10 μm) sandwiched between a pair of plastic films 1006 (having a thickness of, for instance, about 10 μm) which are insoluble in solvents. The aluminum foil 1007 serves to prevent moisture from invading into the battery main body 1003. However, the aluminum foil 1007 is of a thin thickness (about 10 μm), and therefore, there is a tendency in that moisture invasion into the battery main body 1003 cannot be perfectly prevented by the aluminum foil 1007.
The fabrication of a rechargeable lithium battery having such configuration as shown in FIG. 10(a) using aforesaid laminate film 1005 is conducted, for instance, in the following manner. There is provided a laminate film 1005 having a prescribed length. The laminate film 1005 is doubled along a prescribed bending line 1004 to form a folded shape having a space between the two bent laminate films, the battery main body 1003 having the two power output terminals 1001 is installed in said space, and a heat-welded portion 1002 is formed in a peripheral portion of the folded shape having the battery main body 1003 having the two power output terminals 1001 enclosed therein to seal the inside. In this case, the peripheral portion of the folded shape in which the heat-welded portion 1002 is formed comprises the two laminate films 1005 stacked. By heating the peripheral portion while applying a prescribed pressure thereto, the adjacent plastic films 1006 of the two laminate films 1005 are mutually heat-fused and welded to form a heat-welded region 1008. In this case, it is difficult to sufficiently seal neighborhood regions of the two power output terminals 1001. In order to sufficiently seal the neighborhood regions of the two power output terminals 1001, it is necessary to increase the heat-welded portion to an extent which is greater than that required. This situation is liable to entail a problem such that the reliability of the battery is deteriorated. Besides, in general, it is necessary for the heat-welded portion to be provided at a width of 5 mm or more. This situation entails a decrease in the capacity density of the battery. In order to prevent the capacity density of the battery from being decreased, there is considered to adopt a manner of bending also the peripheral portion which is to be heat-welded. However, to bend the peripheral portion involved deteriorates the reliability of the laminate film 1005, where a fear of permitting moisture to be invaded into the battery main body is likely to increase.
Japanese Unexamined Patent Publication No. 213286/1997 (hereinafter referred to as “document 1”) discloses a rechargeable battery which can make up for such shortcomings as above described. In more detail, document 1 discloses a thin rechargeable lithium battery comprising a (a battery main body) installed in a battery vessel formed by molding a thin metal plate, characterized in that said battery vessel has an opening at the face thereof in parallel to said battery main body, a cover plate is disposed at said opening of the battery vessel, and said cover plate is welded to the battery vessel by mean of laser beam welding.
FIG. 11 is a schematic cross-sectional view illustrating the internal constitution of the rechargeable lithium battery disclosed in document 1. In FIG. 11, reference numeral 1100 indicates a battery main body disposed in a thin battery vessel 1105 whose upper face in parallel to said battery main body 1100 has an opening. Reference numeral 1104 indicates a cover plate disposed so as to cover said opening, where the cover plate 1104 is welded to the battery vessel 1105 by means of laser beam irradiation 1108 to seal the inside of the battery vessel 1105. The battery main body 1100 comprises a stacked body comprising a cathode 1101 and an anode 1102 stacked through a separator 1103.
Document 1 describes that according to the technique described therein, it is possible to prepare a thin rechargeable lithium battery having a thickness of 5 mm or less and having a relatively large area.
However, the rechargeable battery having such configuration as shown in FIG. 11 described in document 1 has disadvantages such that because the cover plate 1104 comprises a simple sheet-like plate whose thickness is thin, the cover plate 1104 is insufficient in terms of the physical strength, and because of this, when a stress is vertically or diagonally applied to the battery vessel 1105, the battery vessel is liable to deform, where there is a fear that the cathode and the anode suffer internal shorts. Besides, there is also a disadvantage such that because laser beam is used when the cover plate 1104 is welded to the battery vessel, the battery main body 1100 is unavoidably exposed to heat generated upon the irradiation of the laser beam, and therefore, in order to protect the battery main body from said heat, it is necessary to provide a heat shielding member 1106 between the battery main body 1100 and a welding portion where the cover plate 1104 and the battery vessel 1105 are welded, as shown in FIG. 11. Reference numeral 1107 indicates an interstice formed when the heat shielding member 1106 is provided.
As the heat shielding member 1106, there is used a thermally conductive member having a thickness of about 0.1 mm and which is made of a metallic material having good thermal conductivity such as Cu, Ni or a stainless steel. Because of this, when the anode or the cathode of the battery main body comprises an anode or cathode active material which is liable to expand upon charging or discharging, or when such stress as above described is applied to the battery vessel, the probability of the occurrence of the internal shorts among the cathode and the anode of the battery main body is increased. Further, since the entire thickness of the rechargeable lithium battery is several millimeters, the thickness (about 0.1 mm) of the heat shielding member 1106 is corresponding to several percentages to 5% of the entire thickness of the rechargeable lithium battery, where the capacity density of the rechargeable lithium battery is decreased by a quantity occupied by the heat shielding member 1106. In the case where the heat shielding member 1106 is made to be in a waved form, the capacity density of the rechargeable lithium battery is more decreased.